Avoid redundant redeclaration of GetWarnings(const string&)

std::string GetWarnings(const std::string& strFor) is declared in
warnings.h and defined in warnings.cpp.

src/rpc/mining.cpp

@@ -24,6 +24,7 @@
 #include "util.h"
 #include "utilstrencodings.h"
 #include "validationinterface.h"
+#include "warnings.h"
 
 #include <memory>
 #include <stdint.h>